Large Enhancement of Spin-Flip Scattering Efficiency at Y₃Fe₅O₁₂/Pt Interfaces Due to Vertical Confinement

Magnons, the quanta of spin angular momentum, can be excited in magnetic insulators by spin-flip scattering processes induced by currents applied to a heavy-metal overlayer. The efficiency of generating nonequilibrium magnons is characterized by the interface spin current j_s^int, whose magnitude is considered to depend on the thermal magnon population. Here, we investigate nonlinear magnetoresistance phenomena in Pt arising from current-driven nonequilibrium magnons in Y₃Fe₅O₁₂ (YIG). Remarkably, we find that spin-flip scattering processes are dominated by subthermal magnons at room temperature, resulting in a large modulation of j_s^int with the magnetic field and YIG thickness. Concretely, reducing the YIG thickness from 100 to 10 nm increases j_s^int by a factor ∼20, while increasing the magnetic field exponentially suppresses the magnon generation efficiency. These findings challenge the current understanding on j_s^int and indicate that electrically driven magnonic effects such as damping compensation and magnon condensation can be largely boosted through device miniaturization.